The present invention relates to a multilayer printed circuit board and a method of manufacturing the same, and more particularly, to the manufacture of a thin insulating layer, the formation of conductive holes, and the protection of the circuit board from etching liquids.
In multilayer printed circuit boards, there has been a recent demand to thin insulating layers in order to shorten the distance between pattern layers, which transmit signals at high speeds.
FIG. 1 shows a conventional method for manufacturing a multilayer, printed circuit board. In the method, conductive holes 92 and conductive patterns 93 are formed on insulating substrates 91, and a plural number of the substrates 91 are laminated together.
In the above method, however, the conductive holes 92 and the conductive patterns 93 must be formed beforehand on the insulating substrates 91. This makes it difficult to thin the insulating substrates 91.
FIG. 2 shows a conventional build-up process through which thin insulating layers are formed. In the build-up process, an insulating substrate 91 having a conductive hole 92 and a conductive pattern 93 is prepared. An insulating layer 911, made of prepreg or the like, is laminated on the surface of the substrate 91. A conductive pattern 931 is then formed on the surface of the insulating layer 911. Afterward, the insulating layer 911 is irradiated with ultraviolet rays and developed to form a conductive hole 921 in the insulating layer 911. A plating film 930 is then applied to the wall of the conductive hole 921. Since thin insulating layers are laminated in this method, the distance between the conductive patterns 93, 931 is decreased and high speed transmission of signals is enabled.
In the above build-up process, however, residual resin left on the insulating layer 911 after formation of the conductive hole can result in unsatisfactory conduction of the conductive hole 921. Thus, the conductive hole 921 must be large. However, this makes it difficult to narrow the pitch between conductive holes.
Further, as shown in FIG. 3, in a multilayer printed circuit board having a mounting hole 94, an exposed portion of the conductive pattern 93 in the mounting hole 94 may be corroded by copper foil etching liquid when forming the conductive hole 921. This may result in unsatisfactory connection between the exposed portion of a bonding pad 942 in the mounting hole 94 and a bonding wire.
It is a first object of the present invention to provide a multilayer printed circuit board and a method for manufacturing the same that shortens the distance between patterns and facilitates the formation of minute conductive holes having superior conductive reliability.
It is a second object of the present invention to provide a multilayer electronic component mounting substrate and a method for manufacturing the same that has connection terminals having a superior corrosion resisting characteristic with respect to etching liquids and a superior connecting reliability with respect to bonding wires.
In a first aspect of the present invention, a method for manufacturing a multilayer printed circuit board is provided. First, a core substrate including a core pattern, which has a pad for covering a bottom opening of a conductive hole, is prepared. Then, a laminated plate is formed by laminating an insulating layer on the surface of the core substrate. Then, a surface pattern is formed on the surface of the laminated plate at portions other than where the conductive hole is formed. Then, the conductive hole, the bottom opening of which is covered by the pad, is formed by irradiating a laser beam at a conductive hole formation portion of the laminated plate. Then, the entire surface of the insulating layer, which includes the interior of the conductive hole, is coated with a thin plating film. Subsequently, a mask is applied to the thin plating film with the conductive hole in an opened state and the wall of the conductive hole is coated with a conductive coating. Afterward, the mask is removed. Further, the thin plating film excluding the portions coated by the conductive coating is removed.
The most significant features of the present invention are that a build-up process, which laminates insulating layers on the surface of the core substrate, is performed, and a conductive hole that reaches a pad is milled on a laminated plate by irradiating a laser beam.
In the present invention, a core pattern refers to one layer or two or more layers of conductive patters that are formed on the surface or interior of the core substrate. The surface pattern refers to a conductive pattern formed on the surface of the insulating layer. Further, pattern will refer to the core pattern and/or the surface pattern below.
In this method, the insulating layer is reinforced by the core substrate when forming the conductive hole and the surface pattern. This enables a thinner insulating layer to be formed.
It is preferred that a land surround a middle portion of the conductive hole formation portion. The land and the conductive coating that coats the wall of the conductive hole are both metal and thus have substantially the same coefficient of thermal expansion. This inhibits the conductive coating from falling off the wall of the conductive hole when a thermal impact is applied.
Further, even if the conductive hole is relatively deep, the thin plating film is applied to the wall of the conductive hole in a uniform manner and conduction reliability is improved by providing the land at the middle portion of the conductive hole.
When the land is used only to reinforce the conductive hole, the land and core pattern that are located on the same layer are insulated from each other. However, the land and core pattern located on the same layer may be electrically connected to each other.
It is preferred that the thin plating film have a thickness of 0.01 xcexcm to 5 xcexcm.
It is preferred that the insulative core substrate have a mechanical strength that enables the formation of a pattern and a hole. The core substrate includes a resin substrate filled with glass fiber or glass cloth. The core substrate includes a core pattern formed on at least one of the surface and interior of the core substrate.
It is preferred that the insulating layer have a thickness of 30 xcexcm to 150 xcexcm.
The insulating layer may be formed on either one side or both sides of the core substrate.
Further, the insulating layer may be formed by, for example, printing and applying prepreg, which is formed by semihardening resin impregnated in glass fiber or glass cloth, or applying a sheet of prepreg and then hardening the resin in the prepreg.
It is preferred that the conductive hole have a diameter of 30 xcexcm to 300 xcexcm.
It is preferred that the thin plating film be formed by, for example, a chemical plating film, which is made of conductive material such as copper, tin plating, the application of a solder palladium catalyst, or a lamination of these materials.
A second aspect of the present invention provides a multilayer printed circuit board comprising a core substrate including a core pattern, an insulating layer coating the surface of the core substrate, a surface pattern provided on the surface of the insulating layer, and a conductive hole for electrically connecting the surface pattern to the core pattern. The core pattern includes a pad covering a bottom opening of the conductive hole.
A third aspect of the present invention provides a method for manufacturing a multilayer electronic component mounting substrate. Initially, in a first step, a core substrate including an electronic component mounting hole, a connection terminal exposed together with the mounting hole, and a core pattern, which has a pad for covering a bottom opening of a conductive hole, are prepared. Then, in a second step, a laminated plate is formed by laminating an insulating layer on the surface of the core substrate with the mounting hole and the connection terminal in an exposed state. In a third step, the surface of the connection terminal is coated with an electroless plating film. Then, in a fourth step, a metal layer is formed on the surface of the laminated plate. In a fifth step, the conductive hole, the bottom opening of which is covered by the pad, is formed by irradiating a laser beam at a conductive hole formation portion of the laminated plate. In a sixth step, a conductive coating in the interior of the conductive hole is formed. In a seventh step, the metal layer is etched and the surface pattern is formed. The laminated plate is heated after the third step and before the seventh step.
In the third aspect of the present invention, the connection terminal, which is exposed together with the mounting hole, is coated by the electroless plating film. It is preferred that the connection terminal be made of copper.
However, the copper included in the connection terminal may infiltrate the electroless plating film. Copper is a material that degrades the corrosion characteristic with respect to etching liquids. The electroless plating film is heated to diffuse the copper in the electroless plating film to the film surface. As a result, self-sintering of the electroless plating film is enhanced and a fine film structure is obtained. This improves the corrosion resistance of the electroless plating relative to the etching liquid used to form the surface pattern (seventh process). Accordingly, the connection terminal exposed to the interior of the mounting hole is not corroded by the etching liquid. This improves the bonding strength of bonding wires, flip chips, and soldering connection relative to the connection terminal.
A fourth aspect of the present invention provides a method for manufacturing a multilayer electronic component mounting substrate. Initially, in a first step, a core substrate including an electronic component mounting hole, a connection terminal exposed together with the mounting hole, and a core pattern, which has a pad for covering a bottom opening of a conductive hole, is formed. Then, in a second step, a laminated plate is formed by laminating an insulating layer on the surface of the core substrate with the mounting hole and the connection terminal in an exposed state. In a third step, the surface of the connection terminal is coated with an electroless plating film. Then, in a fourth step, a metal layer is formed on the surface of the laminated plate. In a fifth step, the metal layer is etched to form the surface pattern. Then, in a sixth step, the conductive hole, the bottom opening of which is covered by the pad, is formed by irradiating a laser beam at a conductive hole formation portion of the laminated plate. Then, in a seventh step, a conductive coating in the interior of the conductive hole is formed. The laminated plate is heated after the third step and before the fifth step.
In the third aspect, the conductive hole is formed after the formation of the surface pattern, and in the fourth aspect, the surface pattern is formed after the formation of the conductive hole.
In the fourth aspect, any one of the second and third steps may be performed first. It is only required that the superimposed plate be heated after forming the electroless plating film and before forming the surface pattern.
It is preferred that the electroless plating film be formed by tan electroless Nixe2x80x94Au plating or an electroless Nixe2x80x94Pd plating.
A fifth aspect of the present invention provides a multilayer electronic component mounting substrate comprising a mounting hole for mounting electronic components, a core substrate including a core pattern, an insulating layer arranged on the surface of the core substrate, a surface pattern arranged on the surface of the insulating layer, a conductive hole for electrically connecting the core pattern to the surface pattern, and a connection terminal exposed together with the mounting hole. The connection terminal is coated by an electroless plating film formed by an electroless Nixe2x80x94Au plating or an electroless Nixe2x80x94Pd plating. Further, the core pattern includes a pad covering a bottom opening of the conductive hole.
A sixth aspect of the present invention provides a method for manufacturing a multilayer electronic component mounting substrate. Initially, a core substrate including a core pattern and a mounting hole is prepared. Then, a laminated plate is formed by laminating an insulating layer, which corresponds with the mounting hole, on the surface of the core substrate. The surface of the laminated plate is coated with metal foil to cover the mounting hole. A conductive hole is formed in the laminated plate and the wall of the conductive hole is coated with a conductive coating. The metal foil is patterned to form the surface pattern, and a cover for covering the mounting hole and part of the insulating layer surrounding the mounting hole is formed. Further, the cover is removed by milling part of the insulating layer, which surrounds the mounting hole, to expose the mounting hole.
The most significant features of the present invention are that a build-up process, which laminates insulating layers on the surface of the core substrate, is performed, and the formation of the conductive hole, the coating of the conductive hole with the conductive coating, and the formation of the surface pattern are performed with the mounting hole in a state coated by the metal foil.
A seventh aspect of the present invention provides a multilayer electronic component mounting substrate comprising a core substrate including a core pattern, an insulating layer arranged on the surface of the core substrate, a surface pattern provided on the surface of the insulating layer, a conductive hole electrically connecting the surface pattern and the core pattern, and a mounting hole provided in the core substrate. The insulating layer includes an opening corresponding with the mounting hole and a recess formed surrounding the opening.